The steam generators of pressurized-water nuclear reactors comprise a shell of overall cylindrical shape, inside which a water box is delimited by a thick tube plate to which the bundle of the steam generator tubes is fastened. Each of the tubes in the bundle has two parallel rectilinear legs the end parts of which are fastened inside the tube plate, generally by expansion and by welding.
When the steam generator is in service, the tubes are in contact both with the primary fluid of the nuclear reactor, consisting of pressurized water, and with the secondary fluid consisting of feed water which is vaporized. Under the conditions of use of the steam generator, the tubes are liable to be subjected to various forms of damage and especially to corrosion by the fluids with which they are in contact. Certain tubes of the steam generator may become defective in service and may, for example, exhibit cracks which give rise to leakages resulting in contamination of the secondary fluid.
It is therefore necessary, during the reactor maintenance periods, to intervene in the steam generator and to plug the tubes exhibiting damage capable of giving rise to leakages.
There are various known processes and devices which enable a plug to be placed in the end part of a steam generator tube, from the water box of this steam generator.
It is possible, for example, to use manual plugging tools which require operators to be present inside the water box, which exhibits some degree of radioactivity. The period of intervention in the water box of the steam generator must therefore be strictly limited and monitored.
It is also possible to employ a carrier device of known type which is introduced into the water box of the steam generator and which is capable of installing a tool in the axial extension of the end parts of the tubes to be plugged.
However, there is no known tool permitting the plugging of a tube to be carried out in a completely automatic manner, and remotely controlled.
Furthermore, the known plugging processes and devices of the prior art permit the tube to be closed only in a definite manner without any possibility of subsequent removal of the plug.
In some cases, it may be necessary to carry out the removal of a plug placed in a steam generator tube.
The tubes which are situated in the middle part of the bundle have a curved part connecting their two rectilinear legs with a short radius of curvature. The parts with short radii of curvature, called small bends, render the corresponding tubes particularly sensitive to corrosion because of the presence of a relatively high stress concentration in the curved part.
Programs of blocking the tubes in the middle part of the bundle, in some steam generators have therefore been carried out as a preventive measure.
Since then, effective processes and devices for removing stresses from the small bends have been proposed, and it has become possible to return to use the tubes which had been plugged as a preventive measure.
Furthermore, are also known effective processes for repairing the tubes, which allow the tubes which had been blocked to be brought back into operation, after repair.
It is therefore desirable to have available processes and devices which make it possible to place removable plugs in the tubes of a steam generator and to ensure the withdrawal of these plugs.
There are known plugs for shutting off the tubes of a steam generator which comprise a body of tubular shape, closed by a bottom at one of its ends and a clamping assembly, also tubular, introduced into the bore of the plug body.
The clamping means may comprise, in particular, an insert made of a metal such as nickel and a core made of a nickel alloy with shape memory, both these components being entended to interact in order to ensure the radial expansion of the plug body and its fastening in the tube.
Plugs of this kind can be removed by carrying out the withdrawal of the core, and then the axial withdrawal of the plug body, by applying a pull producing a reduction in area, permitting its removal.
However, there is no known remotely controlled device which makes it possible to carry out both the fitting and the withdrawal of a plug of this kind.